In wireless systems such as orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) wireless systems, contention-based uplink transmission is commonly used for multiple user equipments (UEs) to transmit uplink data to a serving base station (eNB) via a shared uplink channel. For example, a UE may request access and acquire ownership of an uplink access channel to initiate transmission with its serving eNB via an access request procedure such as initial ranging, handover ranging, bandwidth request (BW-REQ) ranging, or random access procedure (RACH). Typically, a contention-based access request procedure consists two phases: a first contention resolution phase and a second request negotiation phase.
FIG. 1 (Prior Art) illustrates a sequence chart of a 5-step BW-REQ ranging procedure used in IEEE 802.16e wireless systems. In the first phase of contention resolution, a UE sends a preamble (e.g., a BW-REQ ranging code) via a shared channel (step 1), and an eNB acknowledges after detecting which UE sends the BW-REQ ranging code (step 2). In the second phase of request negotiation, the UE sends a BW-REQ message for bandwidth allocation (step 3), and the eNB grants uplink resource after correctly decoding the BW-REQ message (step 4). After successful contention and negotiation in both phases, the UE is then able to start scheduled uplink transmission (step 5). Therefore, the request-and-grant mechanism takes five steps and thus is quite time-consuming to complete the entire BW-REQ ranging procedure.
FIG. 2 (Prior Art) illustrates a message sequence chart of a 3-step BW-REQ ranging procedure and a fallback 5-step BW-REQ ranging procedure used in IEEE 802.16m wireless systems. In a 3-step BW-REQ ranging procedure illustrated in the left side of FIG. 2, the UE first sends a BW-REQ ranging code with an embedded BW-REQ message (step 1). The eNB detects the BW-REQ ranging code, decodes the BW-REQ message, and grants uplink resource accordingly (step 2). The UE then starts scheduled uplink data transmission after receiving the UL grant (step 3). By sending the BW-REQ ranging code and the BW-REQ message together, BW-REQ ranging latency is shortened. This 3-step BW-REQ ranging procedure, however, still requires the UE to send the ranging code and ranging message before the scheduled uplink data transmission.
Furthermore, the 3-step BW-REQ ranging procedure falls back to a 5-step BW-REQ ranging procedure when ranging collision occurs. As illustrated in the right side of FIG. 2, the UE first sends a BW-REQ ranging code with an embedded BW-REQ message (step 1). Due to multiple ranging transmission from multiple user equipments (i.e., multiple user equipments send bandwidth requests with different BW-REQ ranging codes and ranging messages at the same time), the BW-REQ messages may not be decodable by the eNB while the BW-REQ ranging codes are decodable (step 2). This is because BW-REQ ranging code is usually more robust than BW-REQ message in the BW-REQ ranging design. As a result, the UE retransmits the BW-REQ message to the eNB (step 3). The eNB decodes the BW-REQ message successfully and grants uplink resource (step 4). Finally, the UE is able to start scheduled uplink data transmission (step 5). Therefore, in the fallback 5-step BW-REQ ranging procedure, access latency is not reduced.